1. Field of the Invention
The present invention relates to a misfire detecting device of a multicylinder internal combustion engine.
2. Description of the Related Art
In a multicylinder internal combustion engine, if one cylinder misfires, the rotational speed of the engine during the explosion stroke of the cylinder which had misfired falls and therefore the time required for the crankshaft to rotate by a certain crank angle during the explosion stroke of the misfiring cylinder becomes longer than that of other cylinders. Accordingly, if the time required for the crankshaft to rotate by a certain crank angle in the explosion stroke of for example the #1 cylinder becomes longer than that of other cylinders, it can be judged that the #1 cylinder has misfired.
When a vehicle is decelerating, however, the time required for the crankshaft to rotate by a certain crank angle continues to increase as well, so the time required for the crankshaft to rotate by a certain crank angle becomes longer than that of other cylinders. Accordingly, as mentioned above, since the time required for the crankshaft to rotate by a certain crank angle becomes longer than that of other cylinders, it is judged that a misfire has occurred. In this case, therefore, a mistaken judgement is made that a misfire has occurred despite no such misfire happening. Therefore, there is known an internal combustion engine which subtracts the increase of time caused by the deceleration operation from the time required for a crankshaft to rotate by a certain crank angle when the vehicle is decelerating and judges that a misfire has occurred when the remaining time has become longer than that of other cylinders (Japanese Unexamined Patent Publication No. 5-52707). In this internal combustion engine, it is possible to detect misfiring with good reliability free from the effect of a deceleration operation even if the vehicle is decelerating.
If, however, a vehicle is being driven on a hilly road and the vehicle is operated at a specific speed where the vehicle as a whole resonates, the slip action of the drive wheels and the grip action of the drive wheels following that will be cyclically repeated and the rotational speed of the engine will fluctuate. Further, the rocking of the engine body will be amplified and it will appear as if the engine rotational speed were fluctuating itself. The fluctuation of the engine rotational speed at this time, however, is different from the fluctuation of the engine rotational speed during a deceleration operation, so as mentioned above the time required for the crankshaft to rotate by a certain crank angle following the firing of a certain cylinder will become longer, temporarily, than that of other cylinders even if the increase of the time caused by the deceleration operation is subtracted from the time required for the crankshaft to rotate by a certain crank angle. Accordingly, it will be erroneously judged that a misfire has occurred even though it has not.